Field of the Invention
The present invention relates to a radiography system applicable to medical equipment, nondestructive testing apparatus, and the like, an X-ray generating apparatus for use in the system, and an X-ray generating tube for use in the apparatus, and more particularly, to an anode thereof.
Description of the Related Art
In an imaging system using an X-ray, in general, an X-ray generating tube is configured to control the trajectory of electrons emitted from a cathode such as a filament with the use of a control electrode, and then accelerate the electrons toward an anode to which an electric potential higher than that of the cathode is applied. The accelerated electrons collide with a target layer formed in the anode, thereby generating an X-ray. The generated X-ray is emitted to the outside of the X-ray generating tube through a transmitting window to irradiate an irradiation object, and the X-ray transmitted through the irradiation object is detected by an X-ray detector, to thereby take a picture of or examine the interior of the irradiation object. If the electrons that collide with the target layer stay in the target layer, the lingering electrons may invite the destabilization of the electron beam trajectory and a drop of the withstand voltage in the X-ray generating tube, which are known as phenomena observed at the time of charging up and in some cases make it difficult to keep the X-ray dose steady. It is therefore a general opinion that the electrons that collide with the target layer need to be led into an electrical conductive path set in advance. An electrical conductive path for electrons is disclosed in Japanese Patent Application Laid-Open No. 2013-51156 in the form of a conductive layer electrically connected to an anode member to which a supporting substrate of a target layer is mounted and to the target layer.
The “X-ray generation efficiency”, an efficiency at which an X-ray is generated by a collision between electrons and a target layer in an X-ray generating tube, is about 1%, and most of the energy input to the target layer is transformed into heat, thus raising temperature in the vicinity of the target layer during the generation of an X-ray. Controlling the generation/non-generation of an X-ray in the course of driving the X-ray generating tube equals controlling the collision of electrons with the target layer, and repeating the generation/non-generation of an X-ray accordingly means repeated rises/drops in the temperature of the target layer. As a result, not only the target layer but also a supporting substrate of the target layer, an anode member, and a conductive layer connected to the target layer and to the anode member repeatedly rise and drop in temperature, and expand and shrink at thermal expansion coefficients of their respective materials. In a configuration disclosed in Japanese Patent Application Laid-Open No. 2013-51156, an end of the conductive layer, which is a thin film, is sandwiched between the anode member, which is a bulk-shaped structural member, and the target supporting substrate. The conductive layer and the anode member are bonded to each other and the conductive layer and the target supporting substrate are bonded to each other in this configuration. The repeated rises/drops in temperature at the end portion therefore causes the concentration of stress and the easing of stress repeatedly on bonding boundaries where the conductive layer is bonded to other members, and may result in a crack in the conductive layer, which is a thin film. When a crack is formed in the conductive layer, the conductivity of the conductive layer drops, and the resultant destabilization of a prescribed anode potential and lingering of electrons in the target layer can present difficulties in keeping the X-ray dose steady.